Screw devices such as a ball screw and a roller screw are provided with a screw shaft, a nut, rolling members such as balls or rollers which are arranged between the screw shaft and the nut, and a circulation component to endlessly circulate the rolling members. The screw device ensures a smooth motion owing to rolling motions of the rolling members, and has been used as a mechanical element which converts a rotational motion to a linear motion or converts a linear motion to a rotational motion.
The screw shaft has an outer circumferential surface on which a helical rolling-member rolling groove is formed. Meanwhile, the nut has an inner circumferential surface on which a helical loaded rolling-member rolling groove is formed so as to face the rolling-member rolling groove of the screw shaft. In a helical loaded rolling-member rolling path provided between the rolling-member rolling groove of the screw shaft and the loaded rolling-member rolling groove of the nut, many rolling members are arranged movably. A circulation component for circulating the rolling members is attached to the nut. The circulation component has a return path which connects one end and the other end of the helical loaded rolling-member rolling groove. Rotating the screw shaft relatively to the nut enables the rolling members to roll and move along the loaded rolling-member rolling path. The rolling members which have rolled to arrive at one end of the loaded rolling-member rolling groove of the nut return to the other end of the loaded rolling-member rolling groove via the return path of the circulation component.
It is usual that, at the boundary between the helical loaded rolling-member rolling path and the return path, the linear return path is arranged to be oriented in a tangential direction of the helical loaded rolling-member rolling path. In other words, it is usually designed such that, when viewed in the axial direction of the nut, the linear return path is directed in the circular loaded rolling-member rolling path. The rolling members move along the helical loaded rolling-member rolling path while being loaded during the movement, and leave the loaded rolling-member rolling path to enter the linear return path, and move along the linear return path.